Structural Analysis and Grouping of Inorganic Matter in Coal by ²⁷Al NMR MQMAS Measurement

Abstract

Structural analysis of inorganic matter in coal is very important, from the viewpoint of both geology and coal utilization. However, it is often difficult to assign inorganic matter from common techniques such as XRD, because some inorganic matter, i.e. clay mineral, in coal has low crystallinity.
On the other hand, solid-state NMR techniques are well suited for the analysis of noncrystalline solids such as inorganic matter in coal. Several kinds of minerals were estimated from the percentage of tetrahedral Al species that was determined by ²⁷Al MAS NMR.
In this study, we successfully developed the method for characterization of 4 minerals (kaolin, montmorillonite, muscovite, and alumina) in coal by virtue of the combination of a high magnetic field (16.4 T) and a MQMAS (Multiple Quantum Magic Angle Spinning) technique that is capable of cancel the second-order quadrupolar broadening. As a result, the structure of the inorganic matter in 26 natural coals was able to be identified. And 26 natural coals could be grouped into 6 groups according to 1-3 types of minerals combination.